Heating of cancerous tumors is now recognized as a valuable adjunct to the long established treatment with chemotherapy or radiotherapy because the treatment effectivity is often enhanced when hyperthermia is included as part of the protocol. It is thus desirable to elevate the tumor temperature as much as possible without causing injury to healthy tissue during the hyperthermia treatment.
Effective heating of a tumor deep within the body, say at 8 to 10 cm depth, has been a goal of many applicator designers. This is very difficult to achieve, however, and is always limited by the allowable temperature elevation of healthy tissue at lessor depths as well as at the muscle-fat interface or at the surface itself.
An overview of a prior art hyperthermia system used to heat tumors in the torso of the body is illustrated in the block diagram of FIG. 1. An RF power source 10 typically provides 400-1000 watts, which is coupled via a matching network 12 and a transmission line 14 to the applicator 16, thence to the torso of the patent 18. Thermometry equipment 20 is connected to the patient to monitor temperature at various locations via fiber-optic probes 22. This thermal information can also be used to control the amplitude of the RF power source through a feed back loop 24 if desired.
Various applicators have been successfully devised to heat tumors. However, heating has most consistently been achieved in surface or near surface tumor therapy where the overlying tissue is not a basic limitation. The design of applicators for this type of therapy are relatively straightforward and often operate at microwave frequencies where some focussing can be achieved. Other applicators that are more specifically designed for deep heating have also been developed. These devices generally operate in the lower HF or VHF frequencies where greater depth of penetration is possible. Several relevant devices of this type are illustrated in FIGS. 2, 3, and 4, and discussed in the literature.
They are:
1. "Deep Heating Electrode," Harrison, U.S. Pat. No. 4,186,729;
2. "Focused Electromagnetic Heating of Muscle Tissue," IEEE Trans. MIT-32, #8, August, 1984, pages 887-888;
3. "Annular Phased Array," IEEE Trans. BME-31, pages 106-114, January, 1984;
4. "A Three-Dimensional Model For the Coaxial TEM Deep-Body Hyperthermia Applicator," Int.J.Hyperthermia, 1986, Vol. 2, No. 3, pages 243-252; and
5. "A New Coaxial TEM Radiofrequency/Microwave Applicator For Non-Invasive Deep-Body Hyperthermia," Journal of Microwave Power, 1983, 18, pages 367-375.
These devices are capable of penetrating the subcutaneous layers and heating imbedded tumor tissue without serious surface overheating. However, each has its limitations.
1. The patent entitled "Deep Heating Electrode," U.S. Pat. No. 4,186,729, illustrated in FIG. 2 consists of a single turn, resonant, non-contacting cylinder 30 that surrounds the body and does not require bolus (water bags) between the electrode and the patient. The conducting sheet forms the inductor 30 and the overlapping sheets form the capacitor 32 required to resonate the circuit. The device typically operates on the lower ISM frequencies, i.e., 13.56, 27.12 or 40.68 MHz. When fed from an RF power source, the resulting induced concentric electric field lines 34 are parallel to the body surface 36 and energy deposition in the deep muscle tissue 38 is not dependent upon electric field lines that must pass through the fat/skin layer 40. Clinical experience with over 1000 patients shows that excessive surface heating is spared and deep heating is often achieved.
However, the concentric electric field strength is proportional to the radius, thus heating is also dependent upon the relative radial location. Calculations and experience have shown that the halfpower depth of penetration is typically 6 to 7 cm below the surface of the the torso with a patient having a 1 to 2 cm fat layer.
2. The paper "Focused Electromagnetic Heating of Muscle Tissue, MIT-32" describes an applicator, as shown in FIG. 3, that consists of two identical metallic cylinders 50 spaced from one another and placed concentrically over a cylindrical phantom simulating muscle tissue 52 to be heated. A very thin 2 mm insulator 54 is placed between the phantom and the metallic cylinders. The cylinder diameter, phantom dimensions and frequency of operation are chosen to obtain construction interference in the central region of the limb/phantom to be heated. For the case cited in this paper, this approach requires an RF power source operating at a frequency of 150 MHz.
The object of the concept illustrated in FIG. 3 is to select a frequency and radial dimension so as to create a radial standing wave in the tissue where the diameter is an even multiple of one-half wavelength in the muscle equivalent tissue. The dielectric constant of muscle tissue is on the order of 80 to 100, thus the wavelength is reduced by the square-root of this value. The electric field from opposite sides is thus reinforced in the central region where preferential localized central heating can occur. The concept is acceptable when working with an experimental uniform cylindrical phantom 52 as shown in FIG. 3. However, the approach has serious limitations when dealing with the shape irregularities of a human torso where the required minimum spacing to the body cannot be maintained and this compromises the necessary radial phase relationship. As discussed therein, a 10 cm diameter phantom was used, with just 2 mm spacing between the phantom and the cylindrical metallic shells, i.e., a very precise spacing not achievable in a clinical environment.
3. The device in the paper "Annular Phased Array," is illustrated in FIG. 4 and consists of a group of as many as 16 dipole elements 60 that are radially spaced around the patient's torso 62 and fed in phase from a common RF source. To obtain sufficient RF coupling to the body, distilled water bags 64 are placed between the dipoles and the patient. This allows the dipole elements to function in a medium having a dielectric constant similar to muscle tissue (approximately 78), thus enhancing the coupling and minimizing the discontinuity between the dipole elements and the body surface. By carefully filling all the voids 66 between the dipole elements and the patient with water bags, efficient RF energy transfer and heating can be achieved at depth.
From a human usage point of view, this device also has serious limitations. It is very difficult to achieve uniform filling of the voids around the patient with water bags. Variable fat thickness, with its lower dielectric constant, also creates additional discontinuities. When these variations occur, localized hot spots will exist that can cause injury or limit the extent of energy input possible without localized thermal damage. It is also very time consuming to properly position the water bags and check for localized heating before treatment begins, thus contributing to patient fatigue and degraded treatment tolerance.
4. The device disclosed in the paper "A Three-Dimensional Model For The Coaxial TEM Deep-Body Hyperthermia Applictor" develops a very detailed three dimensional mathematical model showing that deep heating is possible using a pair of cylindrical sleeves as described above.
5. The device disclosed in the paper "A New Coaxial TEM Radiofrequency/Microwave Applicator For Non-Invasive Deep-Body Hyperthermia" provides a limited theoretical evaluation of the same model showing that the applicator will work with human body dimensions and verifies these predictions with a small model operating at an appropriately scaled higher frequency.
The various prior art devices described above have the limitation of being close fitting around the object heated or using a water bolus to fill the void between the applicator and object to be heated. The IJH paper concludes, "For an efficient electromagnetic coupling, a sufficiently cooled water bolus between the aperture and the human body is necessary." The JMP paper concludes, "To match the patient to the applicator aperture, a distilled water bolus between the patient and the applicator aperture is necessary."
Prior Art devices 2 through 5 are not resonant devices and a serious impedance mismatch with the 50 ohm line to the RF power source will result unless a water bolus is used as described. Moreover, the lack of a resonant structure limits the frequencies which may be employed in the devices.
Accordingly, it is the principal object of the present invention to deposit RF energy in a uniform manner in tissue.
It is another object of the present invention to treat tumors by hyperthermia treatment without the need for a water bolus or an applicator closely fitting around the patient.
Yet another object of the invention is to allow an applicator to function at various frequencies and to optimally couple the RF energy to the applicator.